Codeposited iron and tin electroplate and a process and electroplating bath for its preparation



United States Patent O 3,522,154 CODEPOSITED IRON AND TIN ELECTROPLATEAND A PROCESS AND ELECTROPLATING BATH FOR ITS PREPARATION Donald ArthurSwalheim, Hockessin, Del., assignor to E. I. du Pont de Nemours andCompany, Wilmington, Del., a corporation of Delaware No Drawing. FiledMay 31, 1967, Ser. No. 642,309 Int. Cl. C23b /52, 5/14 U.S. Cl. 204-37ABSTRACT OF THE DISCLOSURE Electrodeposits containing iron and tin on ametal, particularly steel, are plated using an aqueousfluoride-containing bath in which ferrous iron and stannous tin arepresent in controlled amounts. The bath for electroplating the iron andtin codeposit contains a stannous tin concentration of about 0.015 toabout 0.1 mole/1., a mole ratio of ferrous iron to stannous tin havingan upper limit defined by the equation:

and a lower limit defined by the equation:

gr? mole rat1o=0.2 +8.9e'

and a mole ratio of fluoride to ferrous iron plus stannous tin fromabout 1.5 to 7.5.

BACKGROUND OF INVENTION While tinplate is used extensively for packagingvarious types of food, beverages and other products, there is increasingcommercial interest in substituting other coatings for the tin in viewof the relatively high cost of tin. Furthermore, a tin coating is notessential for containers such as beer and soft drink cans since adequateprotection against corrosion is provided by the lacquer. Changingtechniques in fabricating the can are also important factors inselecting substitute coatings for tin. Significant cost reductions havebeen achieved by using nylon-type adhesives instead of solder to bondthe side seam of the body of the can. Tinplate is unsatisfactory foradhesive-bonded side beams because the tin melts at the temperaturesused to apply the adhesive and the bond strength is very low.

One commercial method of producing tinplate having improved corrosionresistance comprises plating the steel with a flash coating of tin (1.2to about 6 millionths inch), refiowing the tin at a temperaturesuflicient to convert it into a flat plate-like iron-tin alloy, thattemperature being between about 490 F. and about 850 -F., electroplatingthe reflowed electrotinned steel with an additional coating of tin andreflowing that additional coating of tin (U.S. Pat. 3,285,833 issuedNov. 15, 1966, to E. R. Morgan et al.). Although tinplate having goodcorrosion resistance is produced by this method, the investment requiredto convert the preplate of tin to FeSn is relatively expensive. Duringthis melting operation, the molten tin reacts with the iron to form aninterlayer consisting of the FeSn intermetallic compound. Theintermetallic compound has been shown to have an important influence onthe protective value of the coating. The extent of coverage of the steelis very important and the degree of coverage is probably associated withthe morphology of the FeSn crystallites. The extent to which completecoverage is achieved depends primarily on the number of nucleationsites, since under normal manufacturing conditions the time availablefor crystal growth is less than 1 sec.

10 Claims 3,522,154 Patented July 28, 1970 No methods of producingsimilar coatings have been disclosed by an electroplating process.Schweikher (U.S. Pat. 2,407,579 issued Sept. 10, 1946) disclosesincluding small amounts of the iron group, e.g., two to ten grams perliter of nickel, cobalt or iron chloride or sulfate, in the plating bathto improve the brightness of the tin deposit. However, it was found thatthese metals do not codeposit to any appreciable extent when present insuch small amounts.

SUMMARY OF THE INVENTION According to the present invention there isprovided a metal having a coating thereon of electrolyticallycodeposited iron and tin which is in intimate adhering contact with themetal, i.e., which is essentially inert when treated anodically in a 5%NaOH solution at a potential not exceeding 0.4 volt. The preferredferrous basis coated prodnot is useful as a semibright,corrosion-resistant container stock (cans), particularly cans having anadhesivebonded side seam, or may be further treated or plated for otherapplications.

The metal products of this invention also comprise the passivatedcodeposit treated electrolytically in chromatetype solutions to improvethe in-storage rust resistance. The metal products also include steelsheet coated with the codeposit or the passivated codeposit and organicsynthetic resins of the type used to coat the interior of cans.

The metal products also include steel sheet having a coating ofelectrolytically codeposited iron and tin and a coating of tin platedover the codeposit which may or may not be heated to melt the tin; ifheated, the reflow should be at a temperature of from about 485 to 720F.

There is also provided a process for preparing electroplated metalarticles which comprises simultaneously comole ratio =0.2+14.7e' set)and a lower limit defined by the equation:

Sn? mole ratio=0.2]8.9e 511+) and the mole ratio of fluoride to ferrousiron plus stannous tin is from about 1.5 to 7.5, preferably about 3 to7.

DETAILED DESCRIPTION OF THE INVENTION The products of this invention maybe prepared by following the general procedures used in Halogen Tinelectroplating described in Schweikher U.S. Pat. 2,407,579. It isessential, however, that the plating bath be modified to control themole ratio of ferrous iron to stannous tin as well as the mole ratio offluoride to total moles of ferrous iron plus stannous tin within thelimits set forth. The mole ratio of ferrous iron to stannous tin willvary depending on the mole concentration of stannous tin in the bath. Inaddition, plating should preferably be carried out at a current densityin excess of about 20 amperes per square foot.

While codeposits of iron and tin are produced over the entire 1.5 to 7.5range of mole ratio of fluoride (F) to total metal (Sn+++Fe++), theappearance of the codeposits improves as the ratio increases. A moleratio of 5:1 is most preferred since no discernible improvement is notedas the ratio is increased from 5:1 to 7.5: 1.

The thickness of the iron-tin codeposit can be readily controlled byvarying the coulombs supplied to the bath as is well known in the artWithin the range of about 5 to 150 coulombs/ftf however, depending uponthe desired end use, the thickness produced is estimated to varry fromabout 0.1x inch to about 4.5 10' inch.

In order to further inhibit corrosion, and especially to retard theformation of oxide films during storage, the codeposited iron-tin platecan be given a typical, electrolytic chromate passivating treatment inan acidic or basic chromate bath. Generally, such baths contain about 1to 75 g./1. of a water-soluble chromate.

The chromate passivating treatment is particularly desirable, althoughnot essential, when preparing a synthetic organic resin coated plateuseful for beverage containers. Among the various organic coatings whichcan be used are resins well documented in the art, but include suchresins as phenolics, expoxies, organosols, vinyls and combinationsthereof, particularly an epoxy phenolic lacquer.

In end uses where tin is necessary, electroplating and reflowing acoating of tin on the codeposit can be effected by conventionaltechniques. Of course, many other of the conventional tin platepractices can be employed such as pickling the base or oiling thecoating.

In practicing the preferred processes of the invention, polyalkyleneglycols of the type described in U.S. Pat. 2,457,152 issued to RaymondA. Hoffman on Dec. 28, 1948, and the patents identified therein areemployed. The polyethylene glycols sold under the trademark Carbowax areemployed in the examples which follow. Useful polyalkylene glycols havenumber average molecular weights from about 1500 to 20,000, preferablyabout 4000 to 20,000 and are employed in the bath at concentrationsranging from about 0.01 to 5 g./l., preferably from about 0.02 to 1g./l.

Example No.

C ontrol.

In addition to the materials described in preparing the baths of thefollowing examples, other materials can be used in combination or inplace of the bath ingredients. The tin salts can be selected fromstannous sulfate, stannous fluoride, or stannous chloride as Well asother watersoluble stannous salts. Likewise, the iron salts can includeferrous chloride as well as ferrous sulfate and other water-solubleferrous salts. To improve conductivity of the bath, such materials assodium chloride or sodium sulfate can be used.

In the examples which follow, the plating characteristics of differentelectrolytes are readily determined in the Hull Cell which has a currentdensity range over the 4-inch wide cathode from essentially zero at thelow end to over 150A/SF at the upper end closest to the anode. Polishedbrass cathodes were used as Hull Cell panels for the tests, and, unlessotherwise indicated, the panels were plated using 3 aniperes for aperiod of 5 seconds. The Du Pont rotating cathode used in some examplesis described by Swalheim in Trans. Electrochem-Soc. 86, 395 (1944).

The examples also use Anodic Stripping to determine whether there is aniron-tin codeposit that is in adhering contact with the metal. In thistest, any free tin present is readily removed from the iron-tincodeposit by treating the plated panel anodically at a controlledpotential not exceeding 0.4 volts in a 5% solution of sodium hydroxide.Current continues to flow until the free tin is completely removed. Theiron-tin codeposit does not dissolve anodically in the caustic solution.Based on the results of the salt spray test and underfilm test, 'whichare de scribed later in the examples, and the above-described AnodicStripping test, the electrolytically deposited coating is distinctlydifferent from substantially pure tin and substantially pure iron. It isa codeposit of iron and tin wherein tin is a major constitoent.

The following examples further illustrate the present invention.

EXAMPLES 1-3 Mole ratio of fluoride to Fe -I-Sn++ Example No ControlControl F- to Fe+++Sn++ Mole Ratio. 511012, g.ll F8804. 7H20, g./l

Na ,g./l- Polyethylene glyeols, g. /l

The appearance of Hull Cell panels plated from the above baths andappearance of the panels after stripping are given in Table II.

TABLE II Mole ratio, Appearance, After Anodic F-/Fe+++Sn++ Appearance,Hull Cell Panels Stripping White Deposit, 0-1A/SF No Deposit, 0-18A/SF.Semi-Bright, 18-36A/SF. Semi-Bright, 18-36A/SF. Dark Streaks, 36150A/SFDark Steaks, 36-150A/SF. White Deposit, 0-18A/SF. No Deposit, O-lSA/SF.

Semi-Bright, 18150/ASF. Dark Deposit, 140-150A/SF White Deposit,O-18A/SF No Deposit, 0-18A/SF.

Semi-Bright, 1890A/SF Semi-Bright, 18-90/ASF. {Dark Deposit, 90-150A/SFDark Deposit, 90-150/ASF. 5 {Semi-Bright, 18150A/SF Semi-Bright,18-150A/SF. 7. 5 Similar to No. 2 Similar to N0. 2.

White Deposit, 0-18A/SF- No Deposit, 0-18A/SF.

3 Semi-Bright, 18-140A/SF It is readily apparent that presence offluoride in the bath is needed in order to produce satisfactorydeposits. Concentrations of fluoride required will vary with the totalmoles of Fe++ and Sn++ present in the bath within the range of about1.521 to 7.5: 1.

EXAMPLES 4-17 Mole ratio of ferrous iron to stannous tin TABLE IIL-BATHCOMPOSITIONS [Varying Mole Ratio of Fe++ to Sn++, Ferrous IronMaintained Constan at 0.17 Mole/1.]

Example 4 5 6 7 Control Control Fe++/Sn++ Mole Ratio 7. 5 5 3. 5 2. 5 2FeSO4. 7H2O, g./l. 48 48 48 48 48 SnCle, g./l 4. 35 6. 5 9. 2 13 16. 3NaI-IFQ, g./l 20. 6 31 3 33. 4 36. 4 39 NaCl, g./l 15 15 15 15 15 15Polyethylene glyeols, 5.11... 0. 38 0.38 0. 38 0. 38 0.38 0.38

Results of Hull Cell plating tests are given in Table IV.

TABLE IV Example Mole Ratio, No. Fe++/Sn++ Appearance, Hull Cell PanelsAppearance, After Anodic Stripping 4 10 Semi-Bright 150A/SF Thin DepositOver Most of Panel. 7 5 {White Deposit, 0-18A/SI No Deposit, 0-18A/SF.

Semi-Bright, 18-150AISF Semi-Bright, 18-150A/SF. 6 5 Similar to No. 5Similar to N o. 5

No Deposit, o-2oA sr. Semi-Bright, 20-150A/SF With Dark Streaks20-40A/SF. No Deposit, 0-24A/SF.

7 3 5 {White Deposit, A7

""""" Semi-Bright, 20-150A/SF tWhite Deposit, 0-24A/SF 5 emi-Bright,24-l50A/SF Semi-Bright, 24-150A/SF With Dark Streaks 24-75A/SF.

D0 2 {White Deposit, 0-30A/SI No Deposit, O-30A/SF.

"- Serni-Bright, 30-150A/SF Semi-Bright, 30-150A/SF With Dark StreaksOver Most of Area.

tin mole ratio was maintained at 5. The composition of With a ferrousiron concentration of about 0.17 mole/l. the baths studied are given inTable VII. good quality codeposits are obtainable over a Fe++ to 15TABLE vr TH COMPOSITIONS Sn++ mole ratio range Of about 10 t0 3. At moleratios [Varying Mole Ratio of Fe++ to Sn++ Stannous Tin MaintainedConstant lower than 3 to 1, the plating range becomes restricted at 0104Mme/L] and the codeposits are somewhat dark in appearance The Example 1314 15 16 17 mole ratio range of 10 to 3 corresponds to a stannous tingeH/SnH M016 Ram 25 5 1 2 BSO47H20,g /l.. 2.9 7.2 14.5 29 58concentration of 0.017 to 0.057 mole/1. SNOIQ, g./l 20 2o 20 20 20 Inthe next series of tests, the stannous tin content NaHFQi 5 20 24 43 Poletii lene 1 eols, .1 0. 38 0.38 0.38 0.38 0. as was maintained constantat 0.052 mole/l. and the mole y y gy g/ Results of the Hull Cell platingtests are shown in Table VIII.

TABLE VIII Example Mole Ratio, No. Fe to Sn++ Appearance, Hull CellPanels Appearanee,AiterAnod1c Stripping 13 0 1 {\NhiteDeposit,036A/SFNoDeposit,045A/SF. Milky Deposit, 36-150AISF Dark Deposit, 45-150. 14 0{White Deposit, 036A/SF No Deposit, 0-36A/SF.

Semi-Bright, 36-150A/SF Semi-Bright With Dark Streaks, 36-150. 15 0 5{White Deposit, 0-36A/SF No Deposit, 0-36A/SF.

Semi-Bright, 36-150A/SF Semi-Bright With Dark Streaks,36150. 16 1 {WhiteDep0sit,0-36A/SF No Deposit,036A/SF.

lSerni-Bright, 36150A/SF Semi-Bright With Dark Streaks, 36-150. 17 2{WhiteDepositfi-SSA/SF. NoDeposit,036A/SF.

------- Serni-Bright, 36150A/SF- Semi-Bright With Dark Streaks, -150.

ratio of Fe++ to Sn++ was varied. The mole ratio of F The results of theplating tests given in Table VIII to moles of F++ plus Sn++ wasmaintained constant at 5. for baths containing 0.104 mole/l. of stannoustin show TABLE V.BATH COMPOSITIONS [Varying Mole Ratio of FE++ to Sn++,Stannous Tin Maintained Constant at 0.052 Mole/1.]

Example No Control Control 8 9 10 11 12 Control Control FE++ sn++Mo1eRatio1 0. 052 o. 125 0. 25 0. 5 1 2 4 e 8 FOSO4-7H20, g.ll 0. 9 1.8 3.67. 2 14. 5 29 5s 87 115 81101), g./l 10 10 1o 10 10 1o 10 1o 10 NaHFe,g./l s. s s. s 9.8 12. 3 1e 24 5e 72 NaCl, g./l 15 15 15 15 15 15 15 1515 P y y g yc -l 38 1 Same. Results of Hull Cell plating tests are givenin Table that the plating range is somewhat restricted and the VI.deposits are somewhat dark after stripping.

TABLE VI Example Mole Ratio No. Fe++ to Sn Appearance, Hull Cell PanelsAppearance, After Anodic Stripping C 1 {White Deposit, 0-30A/SF NoDeposit, 0-36A/SE.

Milk Deposit, 30-120A/SF Dark Deposit, 36-120A/SF.

D 125 {White Deposit, 0-24AlSF No Deposit, 0-30A/SF.

Milky Deposit, 24-130A/sF Dark Deposit, 30120A/SF. 8 0 25 {WhiteDeposit, 0-24A/SF No Deposit, 0-24A/SF.

Semi-Bright, 24135A/SF Semi-Bright, 24-135A/SF With Dark Cast. 9 0 5{White Deposit, 0-24A/SE. No Deposit, 0-24A/SF.

Semi-Bright, 24-140A/SF. Semi-Bright, 24-140A/SF. 0 1 {White Deposit,024A/SF No Deposit, 0 24A/SF. 1 Semi-Bright, 24-140A/SF. Semi-Bright,24-140A/SF. 11 2 {White Deposit, 0-18A/SF. No Deposit, 0-18A/SF.

Semi-Bright, l8l50A/SF Semi-Bright, 18-150A/SF. 2 4 {White Deposit,0-18A/SF. No Deposit, 0-18A/SF. 1 Semi-Bright,l8150A/SF se r ii i iglghg, 18-150A/SE With Dark Streaks,

s C t 1 6 {\VhiteDeposit, 0-18A/SE-.. NoDepesit,024A/SF.

Semi-Bright,18150A/SF Semi-Bright With Dark Streaks, 24-150A/SF;

D 8 {White Deposit, 0i8A/SF 1 No Deposit, 030A/SF.

Milky Deposit, 18150A/SF Semi-Bright With Dark Cast, 30-150A/SF.

In a bath containing 0.052 mole/l. of stannous tin, The results fromTables III through VIII can be sumdeposits having the deslred quahtycharafltenstlcs t marized as expressed in the previously definedequations. Produced Over a ferrous to stannous 1 2 The moleconcentration of stannous tin has a marked 8% aboilt to The preferredrange mo 6 18 effect on the concentration of iron required. At low moleto concentrations of stannous tin in the range of 0.02 to Another seriesof Hull Cell tests were made to determine the effect of the ferrous ironto stannous tin mole the mole who of to stannous can ratio in a bathcontaining 0.104 mole/l. of stannous tin. y Over f broad f Thlsrepresents the p The mole ratio of fluoride to ferrous iron+stannousferred conditions of operation.

7 EXAMPLE 18 Effect of .polyalkylene glycols Hull Cell plating testswere made to determine the effect of polyalkylene glycols on the platingcharacteristics of the electrolyte. The bath compositions are givenbelow:

/S Example 18.-. {White Deposit, O-IBA/SF... No Deposit, -18AISF.

Semi-Bright, l8-150A/SF..- Semi-Bright, l8-150A/SF.

Although codeposits are produced in the absence of polyalkylene glycols,the brightness and general appearance of the codeposits are improved byadding the polyalkylene glycols to the bath.

EXAMPLE 19 Effect of pH on plating characteristics A bath having thefollowing composition was used for this series of tests:

Composition:

SnCl 6.5 g./1. FeSO -7H O-48 g./l. NaHF -3 1.3 g./l.

NaCl-15 1g./l. Polyethylene glycols-0.38 g./l.

The mole ratio of Fe to Sn++ in this bath is 5. The mole ratio of F toFe+++Sn++ is also 5. The pH was changed by either adding NaOH or HCl.The pH was determined colorimetrically using 'benzo yellow indicator.

Brass Hull Cell panels were plated. After plating, the panels weretreated anodically in a NaOH solution. A description of the panelsbefore and after stripping is given in Table IX.

TABLE IX.-EFFECT OF pH ON PLATING Codeposits are produced over the rangeof 2-4 pH. Based on results shown in Table IX, a pH range of 2.5-3.5 ispreferred.

EXAMPLE 20 Effect of temperature on plating characteristics Thecomposition as shown in Example 19 was used in this series of tests.Hull Cell tests were conducted in a similar manner. Results are given inTable X.

TABLE X.-EFFECT OF TEMPERATURE ON PLATING CHARACTERISTICS Appearance ofHull Cell Panels After Anodically Stripping No Deposit, 0-18A/SF Temp,F. As Plated Milky Deposit, 012A/SF "ISeIni-Brrght, 12150A/SFSemi-Bright, 18-150A/SF. {White Deposit, 0-18A/SF No Deposit, 0-18A F.

Semi-Bright, 18l50A/SF Semi-Bright, 18-150A/SF. White Deposit, 0-18 NoDeposit, 0-18.

Semi-Bright, 18150 Semi-Bright, 18-150. {White Deposit, 0-18..- NoDeposit, 0-18.

Semi-Bright, 18-150..- Semi-Bright, 18-150. {Dull White Deposit, 0- NoDeposit, 0-24.

Semi-Bright 24l50 Semi-Bright, 24-150. 200 {Dull White eposit, 0-24 NoDeposit 0-24.

------- Semi-Bright, 24-15o Semi-Bright, 24-150.

The operating temperature range is very broad as shown in Table X, andtherefore not critical. The preferred range is 100-l60 F.

EXAMPLE 21 Plating at high strip speed In most of the commercialoperations involving coating of strip steel with protective coatings,the strip travels through solutions at relatively high strip speeds. Forexample, strip steel is electroplated with tin at speeds up to 2000 ft.min. The Du Pont rotating cathode cell provides a convenient method ofevaluating plating characteristics at high strip speeds. Steel panelswere plated in the cell at 1500 ft./min. using a bath having thefollowing composition:

NaCl-35 g./l.

Polyethylene glycols0.34 g./l. Temperature 150 F.

The time of plating was 0.17 second with a current density of 360amperes per square foot. This quantity of current was equivalent to 60coulombs/sq.ft. The plated deposits were semi-bright in appearance andwere essentially inert when treated anodically in a 5% NaOH solution ata potential not exceeding 0.4 volt.

EXAMPLE 22 chromate-type passivating treatment Protective coatings arefrequently treated in chromatecontaining solutions to passivate thesurface and improve the corrosion resistance. Salt spray tests are usedquite extensively as accelerated tests to measure rust resistance ofcoatings.

Panels plated from the bath of Example 21 were treated cathodically for1 sec. at a current density of 50 A/SF in a solution containing 27.75g./l. CrO and maintained at 150 F. After rinsing and drying, the panelsalong with panels plated with the codeposit but given no chromatetreatment were exposed in a standard 5% NaCl salt spray chamber for 2hours. After exposure for 2 hours, the panels were rinsed with water,dried and examined for rusting. The codeposit treated cathodically inthe chromic acid solution was coated with a very thin film of rust,whereas the codeposit given no chromate passivating treatment washeavily rusted.

EXAMPLE 23 Underfilm corrosion The interior surface of beer and beveragecans is coated with enamel to prevent direct contact of the contentswith the metal. Any significant amount of underfilm corrosion duringstorage cannot be tolerated because this would lead to excessiveexposure of the metal. For example, introduction of a few p.p.m. of ironsalts in beer produces turbidity and has an adverse effect on flavor ofthe beer.

The following procedure was used to evaluate underfilm corrosion. Steelpanels were plated using the following bath:

Bath composition:

SnCl g./l. FeSO -7H O48 g./l. NaHF 45.3 g./1. NaCl-35 g./l. Polyethyleneglycols-0.3 8 g./l. pH2.4

Panels were plated at 360 amp/sq. ft. in the rotating cathode cell at1500 ft./min. Quantity of current applied was 40 coulombs/ sq. ft.

Test strips cut from the panels were coated with a 0.2 mil thick enamel.A Stoner and Mudge No. 2799V enamel, a typical epoxy phenolic lacquerused for beverage containers, was used for these tests. The panels wereheated in an oven maintained at 400 F. for a period of 12 minutes tocure the lacquer.

Strips A x 4" were scribed with an X through the organic coating and thecodeposit thereby exposing the steel basis metal. Strips were thenplaced in a test medium consisting of an aqueous solution containing1.5% NaCl and 1.5% citric acid for 5 days at 80 F. Any undercutting inexcess of 0.1 mm. is considered excessive. Results of these tests showedthat underfilm corrosion for the plated panels was less than 0.1 mm. andsubstantially better than with 0.25# tinplate or plain steel.

EXAMPLE 24 Preplate structure The characteristics of the codeposit wereevaluated by studying the structure using the electron microscope. Abath having the following composition was used to plate the codeposit:

The rotating cathode cell was used to plate the codeposit at a stripspeed of 1300 ft./min. A cathode was plated applying current equivalentto 28 coulombs./ft. After plating, a 2" x 6" strip cut from the platedpanel was heated to a temperature of 540 F. controlling total heat-uptime at 0.3 sec. and allowing a delay time of 0.25 sec. between powershutoff and quenching the strip in water. After heat-treating, the stripwas treated anodically in 5% NaOH at a controlled potential notexceeding 0.4 volt. The current dropped rapidly to zero indicating nosignificant amount of free or uncombined tin in the deposit. An electronphoto micrograph (20,000 X) of the coating showed that the structure ofthe codeposit was extremely fine grained and showed good coverage of thesteel surface.

Another series of panels were made using the rotating cathode cell forplating. The cell was operated at a strip speed of 600 ft./min.Conditions of plating and treating the samples are given in Table XI.

The coulombs/ sq. ft. shown for plating tin deposits a coatingequivalent to a thickness of approximately 45 millionths of an inch.This thickness is commonly referred to as 0.75 lb./base box of 435 sq.ft. of surface.

After plating and refiowing, strips from each panel were treatedanodically in 5% NaOH to remove free tin from the codeposited iron andtin. Electron photomicrographs were then taken of the surfaces tocompare differences in structure. The electron photomicrographs taken at20,000 magnification showed that the structure of the Fe-Sn interlayerfor panel No. 1 in Table XI is coarse grained and typical of reflowedtinplate produced in the conventional manner. An extremely thin preplatelayer of the codeposit prior to plating the tin and refiowing thecomposite coating as on panel No. 2 showed no significant change instructure. However, substantial changes in the structure are shown forthe thicker preplate layers of panels Nos. 35. The electronphotomicrographs for these panels showed a fine-grain type of structurewith good coverage over the steel surface which is highly desirable forsuperior corrosion resistance.

EXAMPLE 25 Effect of inert atmosphere The following bath was used toplate the codeposit: SnCl 5 g./ 1. FeSO -7H O--48 g./1. NaHF 23.2 g./ 1.NaC135 g./1. Polyethylene glycols0.38 g./ 1.

In the first test, the cylinder in the rotating cathode cell wasrevolved at a peripheral speed of 1500 ft./min. Under these conditions,the bath becomes thoroughly entrained with air. The solution wastitrated for SnCl at the start of the test and after 1.5 hrs. ofoperation. Results are shown below:

Intial SnCl 4.93 g./ 1. After 1.5 hrs.2.56 g./ 1.

In the next test, the top of the cell was covered with a split methylmethacrylate cover and N was introduced continuously at a ratesufiicient to exclude air from coming in contact with the bath. Resultsof this test are given below.

Initial SnCl 4.93 g./ 1. After 1.5 hrs-4.93 g./ 1.

This example shows that it is preferred to operate the bath under aninert atmosphere in order to avoid the entrainment of air which oxidizesthe ferrous iron, thus necessitating replenishing the bath with bothstannous tin and ferrous iron.

What is claimed is:

1. A process for preparing a composite coated metal article whichcomprises simultaneously codepositing iron and tin on a metal byelectrodepositing the iron and tin from an aqueous, fluoride containingelectroplating bath having as essential ingredients (1) a stannous tinconcentration of about 0.015 to about 0.1 mole/1., (2) a mole ratio offerrous iron to stannous tin having an upper limit defined by theequation:

Fe++ W mole ratio=0.2+14.7e-

and a lower limit defined by the equation:

2. The process of claim 1 wherein the stannous tin concentration is fromabout 0.02 to about 0.05 mole/l. and the mole ratio of fluoride toferrous iron plus stannous tin is from about 3 to 7.

3. The process of claim 1 wherein the bath additionally contains fromabout 0.01 to about 5 g./l. of polyalkylene glycols having a molecularweight from about 1500 to 20,000.

4. The process of claim 1 wherein the bath temperature is from about toF.

5. The process of claim 1 wherein the bath is maintained under an inertatmosphere.

6. The process of claim 1 wherein the codeposited iron and tin coatingis passivated.

7. The process of claim 1 wherein the codeposited iron and tin coatingis overcoated with an organic synthetic tin.

8. The process of claim 1 wherein the codeposited iron and tin isheat-treated at a temperature of from about 485 to 720 F.

9. An electroplating bath comprising an aqueous fluoride, ferrous ironand stannous tin containing bath having as essential ingredients (1) astannous tin concentration of about 0.015 to about 0.1 mole/1., (2) amole ratio of ferrous iron to stannous tin having an upper limit definedby the equation:

and a lower limit defined by the equation:

12 and (3) a mole ratio of fluoride to ferrous iron plus stannous tinfrom about 1.5 to 7.5, said bath having a pH of about 2 to 4.

10. The electroplating bath of claim 9 wherein the bath additionallycontains from about 0.01 to about 5 g./l. of polyalkylene glycols havinga molecular weight from about 1500 to 20,000.

References Cited UNITED STATES PATENTS 2,407,579 9/1946 Schweikher 204542,758,075 8/1956 Swalheim 204-28 3,082,157 3/1963 Francisco et al 204543,138,548 6/1964 Ham et a1. 20432 3,334,030 8/1967 Notman 20437 J. H.MACK, Primary Examiner W. B. VANSISE, Assistant Examiner U.S. Cl. X.R.

mg UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent: ua gggi r Dated July as, 1970 Inventor-(s) Donald A. Swalheim It is certifiedthat error appears in the above-identified patent and that said LettersPatent are hereby corrected as shown below:

I In Claim 1 at column 10, line 65, the portion after the second formulahas been omitted. Claim 1 in its entirety should read:

1. A process for preparing a composite coated metal article whichcomprises simultaneously codepositing iron and tin on a metal byelectrodepositing the iron and tin from an aqueous, fluoride containingelectroplating bath having as essential ingredients (1) a stannous tinconcentration of about 0.015 to about 0.1 mole/l. (2) a mole ratio offerrous iron to stannous tin having an upper limit defined by theequation:

26(Mole/l. Sn++) W Mole Ratio 0.2 l ifie and a lower limit defined bythe equation:

-72(Mo1e/1. sn g Mole Ratio 0.2 8.9e

and (3) a mole ratio of fluoride to ferrous iron plus stannous tin fromabout 1.5 to 7.5, said bath having a pH of about 2 to I 121 1 mAttestmgofficflr -188mm of Patent.

